1. Field of the Invention
The present invention relates to a signal amplification circuit used in a portable communications device, and more particularly, to a signal amplification circuit whose areas of the top and bottom surfaces are not equal.
2. Description of the Prior Art
Portable communications devices, such as cellular phones, are increasingly popular as they are light weight and do not need a fixed wire for operation. Typically, these devices are used to transmit and receive voice signals. However, as a frequency and power of voice signals is relatively low, the signals must be modulated with much higher frequency carriers to transmit over very long distances. In general, after portable communications devices receive high frequency signals, a series of signal processing steps are required, and the signals must also be amplified, so that users can hear the voice data clearly. Because portable communications devices use wireless high frequency signals as media, the received signals often experience interference in the form of noise. Therefore, each level of amplification circuits in the portable communications devices for amplifying signals must be designed carefully. Impedance matching between each level of amplification circuits must be considered in order to avoid noise propagating and affecting signal amplification. When matching impedance, frequency characteristics of signals are considered to meet different frequency echoes of each level amplification circuits.
Capacitors are commonly used when performing impedance matching. Please refer to FIG. 1. FIG. 1 is a diagram of a structure of a prior art capacitor 10. The capacitor 10 is made of two identically shaped surfaces, a top surface 12 and a bottom surface 16, which are positioned on opposite sides of a dielectric layer 14. The top surface 12 and the bottom surface 16 are thin layer conductors, which attach at both sides of the dielectric layer 14 to form the capacitor 10.
Please refer to FIG. 2. FIG. 2 is a diagram of the capacitor 10 used in a prior art signal amplification circuit 20 to match impedance. The signal amplification circuit 20 is made of an input circuit 22, a pre-amplification circuit 24, the capacitor 10 for matching impedance, a bias voltage circuit 28, four amplification units 30, and an output circuit 32. The input circuit 22 is electrically connected to the pre-amplification circuit 24, and an output port of the pre-amplification circuit 24 is electrically connected to the bottom surface 16 of the capacitor 10 (see FIG. 1). The four amplification units 30 and the bias voltage circuit 28 are electrically connected to the top surface 12 of the capacitor (see FIG.1). Finally, the four amplification units 30 are electrically connected to the output circuit 32.
Operation of the signal amplification circuit 20 is as follows. First, signals are inputted to the pre-amplification circuit 24 from the input circuit 22 and undergo a first amplification in the pre-amplification circuit 24. Next, the signals are transmitted to the bottom surface 16 of the capacitor 10 for impedance matching, passing through the dielectric layer 14 coupled with the top surface 12 of the capacitor 10. After passing through, the signals are distributed to the four amplification units 30 from the top surface 12 to undergo a second signal amplification. After the two signal amplification phases, the signal is outputted to the output circuit 32 to complete the signal amplification function of the signal amplification circuit 20. In the signal amplification circuit 20, the top surface 12 of the capacitor 10 is not only electrically connected to each amplification unit 30, but is also electrically connected to the bias voltage circuit 28. This provides a path of electrical connection so that the bias voltage circuit 28 can provide the power of the bias voltage to each of the amplification units 30 through the top surface 12 of the capacitor 10.
When designing the matching impedance in the prior art, it is often hoped that the capacitance value of the capacitor 10 can change with the need of circuit design. If the capacitance value of the capacitor 10 needs to be changed, the shapes and the areas of the top and bottom surfaces 12, 16 of the capacitor 10 are also changed. This is because the capacitance value of the capacitor 10 and the areas of the top and bottom surfaces 12, 16 of the capacitor 10 are linearly directly proportional. Therefore, in the prior art design, reducing the capacitance value of the capacitor 10 because of the need of impedance matching is usually accomplished by reducing the areas of the top and bottom surfaces 12, 16 of the capacitor 10. However, by reducing the areas of the top and bottom surfaces 12, 16 of the capacitor 10, the electrically connecting path provided to each amplification unit 30 by the bias voltage circuit 28 is reduced. Because each amplification unit 30 in the signal amplification circuit 20 is used for power amplification, the need to bias the voltage power is high. The high power passes through the reduced top surface 12, burning the top surface 12 of the capacitor 10, and the capacitor 10 loses efficacy. To avoid this situation, the top and bottom surfaces 12, 16 of the capacitor 10 are limited to a fixed range. With this constraint, the flexibility of changing the capacitance value of the capacitor 10 reduces, and therefore the difficulty of matching the impedance increases.
It is therefore a primary objective of the claimed invention to provide a signal amplification circuit whose areas of the top and bottom surfaces are not equal to increase the flexibility of designing impedance matching.
In a preferred embodiment, the claimed invention provides a signal amplification circuit used in a portable communications. The signal amplification circuit includes an input circuit for supplying input signals, an output circuit for outputting amplified signals, and a capacitor. The capacitor includes a top surface and a plurality of bottom surfaces, with a total area of the bottom surfaces being less than a total area of the top surface. The bottom surfaces are electrically connected to the input circuit. The signal amplification circuit further includes a plurality of amplification units electrically connected between the top surface and the output circuit and a bias voltage circuit electrically connected to the top surface for supplying a direct current bias voltage to the amplification units. When an input signal passes into the bottom surfaces of the capacitor through the input circuit, the input signal is coupled with the top surface of the capacitor and is passed on to the output circuit through the plurality of amplification units to generate an amplified output signal.
It is an advantage of the claimed invention that the signal amplification circuit increases the flexibility of designing impedance matching.
These and other objectives and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.